All-optical modulation of quantum structure/devices

A new optical modulation approach for multi-band semiconductor devices and its application for laser modulation and imaging are presented. In accordance to numerical calculations, high-speed all-optical amplitude and frequency modulation of quantum cascade laser was achieved for the first experimentally. In addition numerical simulation of a novel high-resolution infrared imaging based on quantum dots offering a unique and inherent spectroscopic feature.Quantum cascade laser have a great potential in communication application due to their intrinsic ultrafast carrier relaxation time allowing modulation up to 100 GHz. However, electrical parasitic effects limit the available bandwidth to below 3 GHz for standard and 10 GHz for specially designed devices, respectively. To realize a parasitic-free modulation, an optical approach is proposed and implemented, using optical excitation of interband transition (in the near infrared spectrum) to control the intersubband lasing emission of a quantum cascade laser (in the middle to far- infrared spectrum). Although limited by the bandwidth of the detecting system circuitry, wavelength conversion and optical amplitude modulation up to 10.35 GHz is demonstrated experimentally, which is far beyond the 2 GHz allowed by quantum cascade laser circuitry. Besides the amplitude modulation, for the first time a 1.66 GHz optical frequency modulation is demonstrated in the emission wavelength of the quantum cascade laser, directly controlled by the near-infrared illumination. Furthermore, different forms of optical switching mechanisms are found in continuous-wave and pulse mode operation, respectively. This optical approach can be used for high-speed modulation, optical switching and wavelength conversion for free space communication.Quantum wells- and quantum dot- based photodetectors are promising candidates for long wavelength radiation detection and imaging based on their high sensitivity. Imaging array with 1 megapixel resolution has recently been demonstrated in quantum well middle infrared photodetectors, yet requires cryogenic cooling for their operation. However, the realization of higher resolution is limited by a dark current and difficulties in corresponding readout circuit. In this work, a novel all-optical readout approach for infrared detection is proposed. It is based on the interaction between interband (visible-near infrared) radiation and intersubband (middle-far infrared) radiation via a three-level system in a quantum dot, utilizing the bottleneck effect caused long electron lifetime in the discrete energy level. Numerical simulation of GaAs/InGaAs quantum dots shows a strong dependency of the interband absorption at the presence of intersubband absorption. This property can be used to convert the middle to far infrared image into a visible-near infrared image, which can be acquired with a commercialized high resolution (up to 10 megapixels) visible-near infrared camera. As the specific conversion scheme allows for high resolution I-to-1 wavelength conversion it allows for direct spectroscopic analysis as well. Without dark current and complex circuitry, this middle-far infrared to visible-near infrared conversion strategy has the possible advantages of low noise, high resolution, and room temperature operation.